I'd want at least 40C. Yeah, it can be debated, but 40C with a 5000mAh battery gives you 200A capacity. With 20C, it's only 100A. Whether or not you'll notice is the part that's debated.

However, I would expect a 40C battery to be more robust internally to handle the additional current, so that's another reason why I like higher rated batteries.

Nothing wrong with Zippy or Turnigy batteries, but I'd go for a 40C rated battery. Both of my Zippys are 40C. At the time that I ordered them, they did not have them in the US. Not sure if they do now or not, though.